BARBULA. 



[ 90 ] 



BARK. 



BAR'BULA, Hedw.— A genus of Pottia- 

 ceous Mosses, synonymous with Tortula, and 

 including some of our commonest mosses, 

 growing on walls Szc. 



BiBL. Wilson, Bryol. Brit. p. 134. 



BARK. — The outer coat of the ti'imks 

 and branches of Dicotyledonous shrubs and 

 trees, succeeding to the epidermis as the 

 young shoots become solid and woody. Bark 

 is a complicated structiu'e, composed of ele- 

 mentary tissues of various characters ; and 

 the great differences of appearance which it 

 presents upon trees which have attained a 

 certain age, residt from the growth and mul- 

 tiplication of the elementary organs being 

 subject to very different laws in different 

 plants. Bark is the collective term applied 

 to the entire cortical mass outside the 

 cambium-region of the stem (Cambium). 

 It contains three distinct regions or forms 

 of structure ; and in young branches the 

 epidermis, still remaining on the outside, 

 constitutes a fourth. 



If we examine a young shoot of the Maple 

 {Acer campestre) while stiU green, by making 

 transverse and perpendicular-radial sections, 

 we find the surface to be covered by an epi- 

 dermis composed of small cells, closely con- 

 joined at their sides. Under this occur six 

 or eight strata of thiu-waHed, colourless 

 cells, which stand vertically over one an- 

 other, and when mature are extended in 

 the radial direction of the branch. These 

 form the cork-substance, sideroiis layer, or 

 periderm. Beneath or within these, we find 

 a layer of thin-walled parenchymatous cells, 

 tilled with protoplasm and chlorophyll 

 granules, forming the cellular envelope, cork- 

 cambium ov jjhellogen. Interposed between 

 the cellular envelope and the cambium region 

 occur the liber-bundles (see Liber), forming 

 X\iQ fibrous layer of the bark. In the bark 

 of the ]\Iaple, the corky substance grows 

 very fast at first, and soon splits the epider- 

 mis above it, but after a certain number of 

 years its growth slackens, so that it seldom 

 acquires verj' great thickness, especially as it 

 is very soft and readily rubbed ofi"; the cel- 

 lular layer does not grow fast, merely keep- 

 ing pace with the enlargement of the stem 

 which it surrounds. The layers of liber 

 increase year by year so as to form a very 

 distinct hbrous layer. 



In the Cork-Oak (Quercus Suber), the 

 bark of which, when young, does not differ 

 much from that of the Maple, the cellular 

 layer grows most in the earlier years, and 

 the epidermis is not destroyed until the third, 



fourth, or fifth ; then the cork-substance 

 begins to increase in an important degree 

 by the multiplication of its cells at the inner 

 side, bordering on the cellular envelope. Xe w 

 layers of cork-cells are produced successively, 

 expanding much in the radial direction. 

 They are thin-walled and destitute of con- 

 tents, of squarish form (PI. 47. figs. 16 SzlT), 

 and soon become dry. The outer lavers 

 being unable to expand sufficiently to allow 

 the enlargement of the stem,tear irregularly, 

 and give the surface of the stem a rough and 

 cracked aspect. On old stems we observe 

 that the formation of these layers has not 

 been continuous, but in successive groups or 

 sets, which causes the appearance of a darker 

 and more solid structm-e, composed of tabu- 

 lar cells, at tlie points where successive sets 

 of layers adjoin, just as is the case at the 

 fines of union of the annual rings of wood in 

 Dicotyledonous stems. But these lines are 

 very irregular. The celUdar envelope takes 

 no shai'e in the formation of the cork of this 

 tree. 



In the Birch (Befula olba), there is a very 

 decided distinction between the layers of the 

 cork-substance, namely between the large 

 thin-waUed colourless cells and the denser 

 tabular cells forming the dark streaks in 

 the cork. The epidermis is succeeded here 

 by a jjericlerm composed of tabular cells 

 with brown contents, corresponding to the 

 darker parts of common cork ; in stems 

 of 20 years' growth, the bark presents as 

 many as fifty lamellae of this substance, 

 which lamellis are separated from each other 

 by layers of the lax white cork-cells. The 

 readiness with which the latter structure 

 gives way causes the lamellas to peel off in 

 thin scales; and these bring away a portion 

 of the white intermediate sti*ucture on both 

 faces, and thus acquire their pecidiar silvery 

 aspect. 



In the Beech (Fayics sylvatica), where the 

 bark is smooth, even on old trees, the growth 

 takes place chiefly in the liber-layers, and 

 the cellular envelope and cork-substance 

 merely expand to make room for the enlarge- 

 ment of the stem ; the cork-substance is 

 here a periderm, i. e. composed of the flat 

 tabular cells, not loose cork tissue. The 

 Holly, Ivy, and other smooth-barked trees 

 are analogous to tliis. 



The scaling ofl"of the bark of the Plane 

 (Plataims occidentalis) arises from the for- 

 mation of layers of tabular peridermal cells 

 between tl)e layers of liber ; the bark out- 

 side the layers dries and falls away by the 

 \ 



